Near-field microwave imaging is a non-ionizing and cost effective sensing modality for variety of applications, including Non-Destructive Evaluation (NDE), medical diagnostics, and detection of concealed weapons. In the latter application, a need exists for a practical system that can image subjects in high foot traffic environments, such as mass transit systems, stadiums, and large public events. In order to handle a potentially constant stream of subjects in motion, the system should measure a subject very quickly (on the order of tens of milliseconds), and reconstruct microwave images of a subject at video rate (e.g., 10 or more frames per second).
The desires to measure subject quickly and reconstruct microwave images at video rates present two major challenges. The first challenge is a cost-effective realization of an electrically large antenna array. Fast acquisition implies that the system should be fully electronic (e.g., it should sample the scene without moving sensors). Furthermore, for proper near-field illumination of a human subject, the array should be roughly the size of the subject (e.g., 1-2 m). For a high resolution system operating in the tens of GHz, this implies an aperture size in excess of 100λ.
A well-known paradigm that mitigates this challenge is multistatic sampling. Such array topologies use transmitters and receivers that are not co-located, and are not separated by a fixed distance. This allows an array with NT transmit elements and NR receive elements to form NTNR spatially diverse samples. This is in contrast to a monostatic sampling scheme (wherein transmitters and receivers are co-located), which uses NTNR transmit-receive elements to achieve the same sampling.
The second challenge is video rate three-dimensional (3D) microwave image formation. The backprojection algorithm can be used with any multistatic configuration, but its computational demands are extreme. Fast Fourier Transform (FFT) imaging has long been used to efficiently construct images sampled with monostatic sampling schemes; however, this formulation cannot be used directly with multistatic sampled data. The modified FFT imaging formulation for multistatic arrays presented in Y. Alvarez et al., “Fourier-based imaging for multistatic radar systems,” IEEE Transactions on Microwave Theory and Techniques, vol. 62, no. 8, pp. 1798-1810, 2014, provides a tremendous improvement over backprojection, but is formulated for topologies where a single transmitter and multiple receivers are used. For topologies with multiple transmitters and receivers, the scheme can be run multiple times, at the expense of processing overhead.